7/25/2005 @ 6:00PM

Love Peptide No. 9

Could a chemical actually make someone fall in lovenot merely infatuation or lust, but lasting love, the kind that forms the foundation of marriages and turns two-timing Lotharios into faithful spouses?

At least in rodents, it seems so.

Mice, rats and rabbits are all famously unfaithful–think about all the sayings about how they multiply. But the prairie vole, a tiny, gray two-ounce rodent, is among the fewer than 5% of mammal species that practice monogamy. Males not only stick with one female, but help care for the young. Larry Young, a researcher at Emory University, has shown that by manipulating two brain hormones, it is possible to change this unique behavior. Blocking the hormone vasopressin makes male voles as unfaithful as rabbits. Blocking another hormone, oxytocin, makes females stop caring about males.

The closely related meadow vole is as promiscuous as Colin Farrell, but by toying with these two hormones, they can be genetically engineered to be faithful. Antisocial mice can be made friendly. There are even hints that the chemicals can have effects in humans.

“It’s only indirect evidence yet,” says Andreas Bartels, whose work has helped show what brain regions are turned on in people who have fallen in love. If oxytocin and vasopressin make animals form long-term relationships, might they not play a similar role in people? “In my view,” he says, “It’s likely that these chemicals would also be involved in the human.”

Indeed, the first evidence that oxytocin is also a powerful chemical in the human brain is infrom a study not of falling in love, but how people handle money.

Oxytocin is already in use as a drug in hospitals in the U.S. to induce labor in women. In Europe, a nose-spray version made by
Novartis
is used to help mothers produce breast milk.

Ernst Fehr, a researcher at the University of Zurich, set out to understand if the spray had any effect on the ability of patients to make economic decisions. “What our research does is it provides a first step into the biology of trust,” says Fehr.

For the study, published in Nature in June, Fehr and two colleagues recruited 128 research subjects. Half were given oxytocin, the other half got placebo. The subjects were then given money and an opportunity to invest it. One group invested their money with another person, dubbed a trustee; a second, with a random gambling mechanism, akin to a slot machine. The investors who got oxytocin entrusted more money to human trustees than those who received placebobut they didn’t take bigger risks when gambling.

Fehr and his colleagues conclude that oxytocin makes people more trusting. Trust, they note, is essential for love, friendship and economic exchange and politics. The same chemical that makes voles mate for the long term also seems to play a role as social glue in people.

However, as a potion to make fools fall in love or to part them from their money, oxytocin turns out to be wanting. For one thing, it must be inhaled in order to get it into the brain. For another, its effects only last for a few hours. What it does represent is a clue as to the brain mechanisms that make people fall in love with one another and stay that way. Understanding the mechanisms behind trust could eventually even have medical effects. In particular, researchers point to diseases such as autism and schizophrenia, where the patients’ ability to form real bonds with others seems to some degree faulty.

As for love, other brain chemicals are likely to be involved as well, such as dopaminethe brain’s main pleasure chemical. Levels of the brain chemical serotonin also seem to drop in the newly infatuated. When people fall in love, apparently, there’s a lot going on in their minds.

“Humans are much more complex than animals,” says Bartels, “so it’s unlikely that you could just give a human a drug and he would fall in love. There are many other factors that influence that. But it might lower your threshold in some way.

According to Bartels, “It is difficult to predict what the outcome would be.”